Advances in micro-electromechanical technology have enabled the fabrication of microfluidic devices integrated with various functions that can be implemented in applications related to biology and medicine. A microfluidic device is an instrument that can control the behavior of very small amounts of fluid (e.g., such as nL, pL, and fL) through channels with dimensions in relatively small dimensions, e.g., the sub-millimeter range. Microfluidic devices can be implemented to obtain a variety of analytical measurements including molecular diffusion values, chemical binding coefficients, pH values, fluid viscosity, molecular reaction kinetics, etc. Microfluidic devices can be built on microchips to detect, separate and analyze biological samples, which can also be referred to as a lab-on-a-chip. For example, a microfluidic device may use body fluids or solutions containing cells or cell parts to diagnose diseases. Inside microfluidic channels of, for example, a microfluidic flow cytometer, biological particles (e.g., including cells, beads, and macromolecules) can be interrogated according to their optical, electrical, acoustic, and magnetic responses. The positions of the samples within the microchannels can significantly affect the quality of signals, e.g., such as the value of coefficient of variation (CV).
Some techniques for positioning particles or samples inside microchannels (e.g., controlling and manipulating their positions) can be categorized into sheath and sheathless focusing approaches. Sheath focusing employs sheath fluids to narrow sample flows. Although this approach can be effective, the design can have a relatively low throughput which can become a limiting factor for certain applications. For example, for many microfluidic devices, sheath flow can confine the particles in the in-plane direction, but can require special designs and more complex processes to achieve particle confinement in the out-of-plane direction. Sheathless approaches can increase the throughput for applications that require a large volume of samples, e.g., such as milliliters of whole blood or body fluid. For example, to control particle behaviors in a sheathless design, an external force can be applied to move particles to the reestablished equilibrium positions through dielectrophoretic (DEP) effects, acoustic effects, or inertial effects induced by the balance between lifting and drag forces. For example, microfluidic devices using inertial focusing can be simple to fabricate because inertial focusing does not require additional electrodes and external signals to guide the suspended particles in the microfluidic channels.